[MERGE][#295161] Ensure valid messages and revision properties

[Aaron Bentley]

John Arbash Meinel wrote:
> At the moment, we have a small issue with how we (de)serialize commit
> messages and revision properties because we go through XML.
> Specifically, when writing, we are allowed to write things like "\r" and
> "\r\n" to the string, but when those strings get read back, they will
> all be normalized to "\n". (This happens as part of the Expat parsing.)

To be clear: you want to forbid \r both in XML markup and in XML tag
bodies?  That seems to impair our ability to do lossless conversions.

Aaron